PskKeyManager

Class Overview

Provider of key material for pre-shared key (PSK) key exchange used in TLS-PSK cipher suites.

Overview of TLS-PSK

TLS-PSK is a set of TLS/SSL cipher suites which rely on a symmetric pre-shared key (PSK) to secure the TLS/SSL connection and mutually authenticate its peers. These cipher suites may be a more natural fit compared to conventional public key based cipher suites in some scenarios where communication between peers is bootstrapped via a separate step (for example, a pairing step) and requires both peers to authenticate each other. In such scenarios a symmetric key (PSK) can be exchanged during the bootstrapping step, removing the need to generate and exchange public key pairs and X.509 certificates.

When a TLS-PSK cipher suite is used, both peers have to use the same key for the TLS/SSL handshake to succeed. Thus, both peers are implicitly authenticated by a successful handshake. This removes the need to use a TrustManager in conjunction with this KeyManager.

Supporting multiple keys

A peer may have multiple keys to choose from. To help choose the right key, during the handshake the server can provide a PSK identity hint to the client, and the client can provide a PSK identity to the server. The contents of these two pieces of information are specific to application-level protocols.

NOTE: Both the PSK identity hint and the PSK identity are transmitted in cleartext. Moreover, these data are received and processed prior to peer having been authenticated. Thus, they must not contain or leak key material or other sensitive information, and should be treated (e.g., parsed) with caution, as untrusted data.

The high-level flow leading to peers choosing a key during TLS/SSL handshake is as follows:

1. Server receives a handshake request from client.
2. Server replies, optionally providing a PSK identity hint to client.
3. Client chooses the key.
4. Client provides a PSK identity of the chosen key to server.
5. Server chooses the key.

In the flow above, either peer can signal that they do not have a suitable key, in which case the the handshake will be aborted immediately. This may enable a network attacker who does not know the key to learn which PSK identity hints or PSK identities are supported. If this is a concern then a randomly generated key should be used in the scenario where no key is available. This will lead to the handshake aborting later, due to key mismatch -- same as in the scenario where a key is available -- making it appear to the attacker that all PSK identity hints and PSK identities are supported.

Maximum sizes

The maximum supported sizes are as follows:

• 256 bytes for keys (see MAX_KEY_LENGTH_BYTES),
• 128 bytes for PSK identity and PSK identity hint (in modified UTF-8 representation) (see MAX_IDENTITY_LENGTH_BYTES and MAX_IDENTITY_HINT_LENGTH_BYTES).

Subclassing

Subclasses should normally provide their own implementation of getKey because the default implementation returns no key, which aborts the handshake.

Known issues

The implementation of ECDHE_PSK cipher suites in API Level 21 contains a bug which breaks compatibility with other implementations. ECDHE_PSK cipher suites are enabled by default on platforms with API Level 21 when an SSLContext is initialized with a PskKeyManager. A workaround is to disable ECDHE_PSK cipher suites on platforms with API Level 21.

Example

The following example illustrates how to create an SSLContext which enables the use of TLS-PSK in SSLSocket, SSLServerSocket and SSLEngine instances obtained from it.

 PskKeyManager pskKeyManager = ...;

 SSLContext sslContext = SSLContext.getInstance("TLS");
 sslContext.init(
         new KeyManager[] {pskKeyManager},
         new TrustManager[0], // No TrustManagers needed for TLS-PSK
         null // Use the default source of entropy
         );

 SSLSocket sslSocket = (SSLSocket) sslContext.getSocketFactory().createSocket(...);
 

Summary

Constants
int	MAX_IDENTITY_HINT_LENGTH_BYTES	Maximum supported length (in bytes) for PSK identity hint (in modified UTF-8 representation).
int	MAX_IDENTITY_LENGTH_BYTES	Maximum supported length (in bytes) for PSK identity (in modified UTF-8 representation).
int	MAX_KEY_LENGTH_BYTES	Maximum supported length (in bytes) for PSK.

Public Constructors
	PskKeyManager()

Public Methods
String	chooseClientKeyIdentity(String identityHint, SSLEngine engine) Gets the PSK identity to report to the server to help agree on the PSK for the provided engine.
String	chooseClientKeyIdentity(String identityHint, Socket socket) Gets the PSK identity to report to the server to help agree on the PSK for the provided socket.
String	chooseServerKeyIdentityHint(Socket socket) Gets the PSK identity hint to report to the client to help agree on the PSK for the provided socket.
String	chooseServerKeyIdentityHint(SSLEngine engine) Gets the PSK identity hint to report to the client to help agree on the PSK for the provided engine.
SecretKey	getKey(String identityHint, String identity, SSLEngine engine) Gets the PSK to use for the provided engine.
SecretKey	getKey(String identityHint, String identity, Socket socket) Gets the PSK to use for the provided socket.

[Expand] Inherited Methods

From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals(Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Invoked when the garbage collector has detected that this instance is no longer reachable. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.